The sigaction() system call is used to change the action taken by a
process on receipt of a specific signal. (See signal(7) for an
overview of signals.)
signum specifies the signal and can be any valid signal except
SIGKILL and SIGSTOP.
If act is non-NULL, the new action for signal signum is installed
from act. If oldact is non-NULL, the previous action is saved in
oldact.
The sigaction structure is defined as something like:
struct sigaction {
void (*sa_handler)(int);
void (*sa_sigaction)(int, siginfo_t *, void *);
sigset_t sa_mask;
int sa_flags;
void (*sa_restorer)(void);
};
On some architectures a union is involved: do not assign to both
sa_handler and sa_sigaction.
The sa_restorer field is not intended for application use. (POSIX
does not specify a sa_restorer field.) Some further details of pur‐
pose of this field can be found in sigreturn(2).
sa_handler specifies the action to be associated with signum and may
be SIG_DFL for the default action, SIG_IGN to ignore this signal, or
a pointer to a signal handling function. This function receives the
signal number as its only argument.
If SA_SIGINFO is specified in sa_flags, then sa_sigaction (instead of
sa_handler) specifies the signal-handling function for signum. This
function receives the signal number as its first argument, a pointer
to a siginfo_t as its second argument and a pointer to a ucontext_t
(cast to void *) as its third argument. (Commonly, the handler func‐
tion doesn't make any use of the third argument. See getcontext(3)
for further information about ucontext_t.)
sa_mask specifies a mask of signals which should be blocked (i.e.,
added to the signal mask of the thread in which the signal handler is
invoked) during execution of the signal handler. In addition, the
signal which triggered the handler will be blocked, unless the
SA_NODEFER flag is used.
sa_flags specifies a set of flags which modify the behavior of the
signal. It is formed by the bitwise OR of zero or more of the fol‐
lowing:
SA_NOCLDSTOP
If signum is SIGCHLD, do not receive notification when
child processes stop (i.e., when they receive one of
SIGSTOP, SIGTSTP, SIGTTIN, or SIGTTOU) or resume (i.e.,
they receive SIGCONT) (see wait(2)). This flag is mean‐
ingful only when establishing a handler for SIGCHLD.
SA_NOCLDWAIT (since Linux 2.6)
If signum is SIGCHLD, do not transform children into zom‐
bies when they terminate. See also waitpid(2). This flag
is meaningful only when establishing a handler for
SIGCHLD, or when setting that signal's disposition to
SIG_DFL.
If the SA_NOCLDWAIT flag is set when establishing a han‐
dler for SIGCHLD, POSIX.1 leaves it unspecified whether a
SIGCHLD signal is generated when a child process termi‐
nates. On Linux, a SIGCHLD signal is generated in this
case; on some other implementations, it is not.
SA_NODEFER
Do not prevent the signal from being received from within
its own signal handler. This flag is meaningful only when
establishing a signal handler. SA_NOMASK is an obsolete,
nonstandard synonym for this flag.
SA_ONSTACK
Call the signal handler on an alternate signal stack pro‐
vided by sigaltstack(2). If an alternate stack is not
available, the default stack will be used. This flag is
meaningful only when establishing a signal handler.
SA_RESETHAND
Restore the signal action to the default upon entry to the
signal handler. This flag is meaningful only when estab‐
lishing a signal handler. SA_ONESHOT is an obsolete, non‐
standard synonym for this flag.
SA_RESTART
Provide behavior compatible with BSD signal semantics by
making certain system calls restartable across signals.
This flag is meaningful only when establishing a signal
handler. See signal(7) for a discussion of system call
restarting.
SA_RESTORERNot intended for application use. This flag is used by C
libraries to indicate that the sa_restorer field contains
the address of a "signal trampoline". See sigreturn(2)
for more details.
SA_SIGINFO (since Linux 2.2)
The signal handler takes three arguments, not one. In
this case, sa_sigaction should be set instead of sa_han‐dler. This flag is meaningful only when establishing a
signal handler.
The siginfo_t argument to a SA_SIGINFO handler
When the SA_SIGINFO flag is specified in act.sa_flags, the signal
handler address is passed via the act.sa_sigaction field. This han‐
dler takes three arguments, as follows:
void
handler(int sig, siginfo_t *info, void *ucontext)
{
...
}
The siginfo_t data type is a structure with the following fields:
siginfo_t {
int si_signo; /* Signal number */
int si_errno; /* An errno value */
int si_code; /* Signal code */
int si_trapno; /* Trap number that caused
hardware-generated signal
(unused on most architectures) */
pid_t si_pid; /* Sending process ID */
uid_t si_uid; /* Real user ID of sending process */
int si_status; /* Exit value or signal */
clock_t si_utime; /* User time consumed */
clock_t si_stime; /* System time consumed */
sigval_t si_value; /* Signal value */
int si_int; /* POSIX.1b signal */
void *si_ptr; /* POSIX.1b signal */
int si_overrun; /* Timer overrun count;
POSIX.1b timers */
int si_timerid; /* Timer ID; POSIX.1b timers */
void *si_addr; /* Memory location which caused fault */
long si_band; /* Band event (was int in
glibc 2.3.2 and earlier) */
int si_fd; /* File descriptor */
short si_addr_lsb; /* Least significant bit of address
(since Linux 2.6.32) */
void *si_lower; /* Lower bound when address violation
occurred (since Linux 3.19) */
void *si_upper; /* Upper bound when address violation
occurred (since Linux 3.19) */
int si_pkey; /* Protection key on PTE that caused
fault (since Linux 4.6) */
void *si_call_addr; /* Address of system call instruction
(since Linux 3.5) */
int si_syscall; /* Number of attempted system call
(since Linux 3.5) */
unsigned int si_arch; /* Architecture of attempted system call
(since Linux 3.5) */
}
si_signo, si_errno and si_code are defined for all signals.
(si_errno is generally unused on Linux.) The rest of the struct may
be a union, so that one should read only the fields that are meaning‐
ful for the given signal:
* Signals sent with kill(2) and sigqueue(3) fill in si_pid and
si_uid. In addition, signals sent with sigqueue(3) fill in si_int
and si_ptr with the values specified by the sender of the signal;
see sigqueue(3) for more details.
* Signals sent by POSIX.1b timers (since Linux 2.6) fill in si_over‐run and si_timerid. The si_timerid field is an internal ID used by
the kernel to identify the timer; it is not the same as the timer
ID returned by timer_create(2). The si_overrun field is the timer
overrun count; this is the same information as is obtained by a
call to timer_getoverrun(2). These fields are nonstandard Linux
extensions.
* Signals sent for message queue notification (see the description of
SIGEV_SIGNAL in mq_notify(3)) fill in si_int/si_ptr, with the
sigev_value supplied to mq_notify(3); si_pid, with the process ID
of the message sender; and si_uid, with the real user ID of the
message sender.
* SIGCHLD fills in si_pid, si_uid, si_status, si_utime, and si_stime,
providing information about the child. The si_pid field is the
process ID of the child; si_uid is the child's real user ID. The
si_status field contains the exit status of the child (if si_code
is CLD_EXITED), or the signal number that caused the process to
change state. The si_utime and si_stime contain the user and sys‐
tem CPU time used by the child process; these fields do not include
the times used by waited-for children (unlike getrusage(2) and
times(2)). In kernels up to 2.6, and since 2.6.27, these fields
report CPU time in units of sysconf(_SC_CLK_TCK). In 2.6 kernels
before 2.6.27, a bug meant that these fields reported time in units
of the (configurable) system jiffy (see time(7)).
* SIGILL, SIGFPE, SIGSEGV, SIGBUS, and SIGTRAP fill in si_addr with
the address of the fault. On some architectures, these signals
also fill in the si_trapno field.
Some suberrors of SIGBUS, in particular BUS_MCEERR_AO and
BUS_MCEERR_AR, also fill in si_addr_lsb. This field indicates the
least significant bit of the reported address and therefore the
extent of the corruption. For example, if a full page was cor‐
rupted, si_addr_lsb contains log2(sysconf(_SC_PAGESIZE)). When
SIGTRAP is delivered in response to a ptrace(2) event
(PTRACE_EVENT_foo), si_addr is not populated, but si_pid and si_uid
are populated with the respective process ID and user ID responsi‐
ble for delivering the trap. In the case of seccomp(2), the tracee
will be shown as delivering the event. BUS_MCEERR_* and
si_addr_lsb are Linux-specific extensions.
The SEGV_BNDERR suberror of SIGSEGV populates si_lower and
si_upper.
The SEGV_PKUERR suberror of SIGSEGV populates si_pkey.
* SIGIO/SIGPOLL (the two names are synonyms on Linux) fills in
si_band and si_fd. The si_band event is a bit mask containing the
same values as are filled in the revents field by poll(2). The
si_fd field indicates the file descriptor for which the I/O event
occurred; for further details, see the description of F_SETSIG in
fcntl(2).
* SIGSYS, generated (since Linux 3.5) when a seccomp filter returns
SECCOMP_RET_TRAP, fills in si_call_addr, si_syscall, si_arch,
si_errno, and other fields as described in seccomp(2).
The si_code field
The si_code field inside the siginfo_t argument that is passed to a
SA_SIGINFO signal handler is a value (not a bit mask) indicating why
this signal was sent. For a ptrace(2) event, si_code will contain
SIGTRAP and have the ptrace event in the high byte:
(SIGTRAP | PTRACE_EVENT_foo << 8).
For a non-ptrace(2) event, the values that can appear in si_code are
described in the remainder of this section. Since glibc 2.20, the
definitions of most of these symbols are obtained from <signal.h> by
defining feature test macros (before including any header file) as
follows:
* _XOPEN_SOURCE with the value 500 or greater;
* _XOPEN_SOURCE and _XOPEN_SOURCE_EXTENDED; or
* _POSIX_C_SOURCE with the value 200809L or greater.
For the TRAP_* constants, the symbol definitions are provided only in
the first two cases. Before glibc 2.20, no feature test macros were
required to obtain these symbols.
For a regular signal, the following list shows the values which can
be placed in si_code for any signal, along with the reason that the
signal was generated.
SI_USERkill(2).
SI_KERNEL
Sent by the kernel.
SI_QUEUEsigqueue(3).
SI_TIMER
POSIX timer expired.
SI_MESGQ (since Linux 2.6.6)
POSIX message queue state changed; see mq_notify(3).
SI_ASYNCIO
AIO completed.
SI_SIGIO
Queued SIGIO (only in kernels up to Linux 2.2; from Linux
2.4 onward SIGIO/SIGPOLL fills in si_code as described
below).
SI_TKILL (since Linux 2.4.19)
tkill(2) or tgkill(2).
The following values can be placed in si_code for a SIGILL signal:
ILL_ILLOPC
Illegal opcode.
ILL_ILLOPN
Illegal operand.
ILL_ILLADR
Illegal addressing mode.
ILL_ILLTRP
Illegal trap.
ILL_PRVOPC
Privileged opcode.
ILL_PRVREG
Privileged register.
ILL_COPROC
Coprocessor error.
ILL_BADSTK
Internal stack error.
The following values can be placed in si_code for a SIGFPE signal:
FPE_INTDIV
Integer divide by zero.
FPE_INTOVF
Integer overflow.
FPE_FLTDIV
Floating-point divide by zero.
FPE_FLTOVF
Floating-point overflow.
FPE_FLTUND
Floating-point underflow.
FPE_FLTRES
Floating-point inexact result.
FPE_FLTINV
Floating-point invalid operation.
FPE_FLTSUB
Subscript out of range.
The following values can be placed in si_code for a SIGSEGV signal:
SEGV_MAPERR
Address not mapped to object.
SEGV_ACCERR
Invalid permissions for mapped object.
SEGV_BNDERR (since Linux 3.19)
Failed address bound checks.
SEGV_PKUERR (since Linux 4.6)
Access was denied by memory protection keys. See
pkeys(7). The protection key which applied to this access
is available via si_pkey.
The following values can be placed in si_code for a SIGBUS signal:
BUS_ADRALN
Invalid address alignment.
BUS_ADRERR
Nonexistent physical address.
BUS_OBJERR
Object-specific hardware error.
BUS_MCEERR_AR (since Linux 2.6.32)
Hardware memory error consumed on a machine check; action
required.
BUS_MCEERR_AO (since Linux 2.6.32)
Hardware memory error detected in process but not con‐
sumed; action optional.
The following values can be placed in si_code for a SIGTRAP signal:
TRAP_BRKPT
Process breakpoint.
TRAP_TRACE
Process trace trap.
TRAP_BRANCH (since Linux 2.4, IA64 only))
Process taken branch trap.
TRAP_HWBKPT (since Linux 2.4, IA64 only))
Hardware breakpoint/watchpoint.
The following values can be placed in si_code for a SIGCHLD signal:
CLD_EXITED
Child has exited.
CLD_KILLED
Child was killed.
CLD_DUMPED
Child terminated abnormally.
CLD_TRAPPED
Traced child has trapped.
CLD_STOPPED
Child has stopped.
CLD_CONTINUED (since Linux 2.6.9)
Stopped child has continued.
The following values can be placed in si_code for a SIGIO/SIGPOLL
signal:
POLL_IN
Data input available.
POLL_OUT
Output buffers available.
POLL_MSG
Input message available.
POLL_ERR
I/O error.
POLL_PRI
High priority input available.
POLL_HUP
Device disconnected.
The following value can be placed in si_code for a SIGSYS signal:
SYS_SECCOMP (since Linux 3.5)
Triggered by a seccomp(2) filter rule.

EFAULT act or oldact points to memory which is not a valid part of
the process address space.
EINVAL An invalid signal was specified. This will also be generated
if an attempt is made to change the action for SIGKILL or
SIGSTOP, which cannot be caught or ignored.

A child created via fork(2) inherits a copy of its parent's signal
dispositions. During an execve(2), the dispositions of handled
signals are reset to the default; the dispositions of ignored signals
are left unchanged.
According to POSIX, the behavior of a process is undefined after it
ignores a SIGFPE, SIGILL, or SIGSEGV signal that was not generated by
kill(2) or raise(3). Integer division by zero has undefined result.
On some architectures it will generate a SIGFPE signal. (Also
dividing the most negative integer by -1 may generate SIGFPE.)
Ignoring this signal might lead to an endless loop.
POSIX.1-1990 disallowed setting the action for SIGCHLD to SIG_IGN.
POSIX.1-2001 and later allow this possibility, so that ignoring
SIGCHLD can be used to prevent the creation of zombies (see wait(2)).
Nevertheless, the historical BSD and System V behaviors for ignoring
SIGCHLD differ, so that the only completely portable method of
ensuring that terminated children do not become zombies is to catch
the SIGCHLD signal and perform a wait(2) or similar.
POSIX.1-1990 specified only SA_NOCLDSTOP. POSIX.1-2001 added
SA_NOCLDSTOP, SA_NOCLDWAIT, SA_NODEFER, SA_ONSTACK, SA_RESETHAND,
SA_RESTART, and SA_SIGINFO. Use of these latter values in sa_flags
may be less portable in applications intended for older UNIX
implementations.
The SA_RESETHAND flag is compatible with the SVr4 flag of the same
name.
The SA_NODEFER flag is compatible with the SVr4 flag of the same name
under kernels 1.3.9 and newer. On older kernels the Linux
implementation allowed the receipt of any signal, not just the one we
are installing (effectively overriding any sa_mask settings).
sigaction() can be called with a NULL second argument to query the
current signal handler. It can also be used to check whether a given
signal is valid for the current machine by calling it with NULL
second and third arguments.
It is not possible to block SIGKILL or SIGSTOP (by specifying them in
sa_mask). Attempts to do so are silently ignored.
See sigsetops(3) for details on manipulating signal sets.
See signal-safety(7) for a list of the async-signal-safe functions
that can be safely called inside from inside a signal handler.
C library/kernel differences
The glibc wrapper function for sigaction() gives an error (EINVAL) on
attempts to change the disposition of the two real-time signals used
internally by the NPTL threading implementation. See nptl(7) for
details.
The original Linux system call was named sigaction(). However, with
the addition of real-time signals in Linux 2.2, the fixed-size,
32-bit sigset_t type supported by that system call was no longer fit
for purpose. Consequently, a new system call, rt_sigaction(), was
added to support an enlarged sigset_t type. The new system call
takes a fourth argument, size_t sigsetsize, which specifies the size
in bytes of the signal sets in act.sa_mask and oldact.sa_mask. This
argument is currently required to have the value sizeof(sigset_t) (or
the error EINVAL results). The glibc sigaction() wrapper function
hides these details from us, transparently calling rt_sigaction()
when the kernel provides it.
Undocumented
Before the introduction of SA_SIGINFO, it was also possible to get
some additional information, namely by using a sa_handler with second
argument of type struct sigcontext. See the relevant Linux kernel
sources for details. This use is obsolete now.

In kernels up to and including 2.6.13, specifying SA_NODEFER in
sa_flags prevents not only the delivered signal from being masked
during execution of the handler, but also the signals specified in
sa_mask. This bug was fixed in kernel 2.6.14.

This page is part of release 4.13 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2017-09-15 SIGACTION(2)